Field of the Invention
The invention relates to a method in which a semiconductor structure is produced on a substrate. A protective layer is applied to the semiconductor structure in order to protect the semiconductor structure from being impaired during subsequent processing steps. At least one intermediate process is carried out for further fabrication of the semiconductor structure or for fabrication of a further semiconductor structure that is to be formed on the substrate. The protective layer remains on the semiconductor structure or is removed from the semiconductor structure that is to be protected, for example in order for it to be structured further.
The substrate is, for example, a semiconductor wafer. However, ceramic substrates or thin-film substrates are also used.
One process is the processing of the substrate under constant process conditions in a processing installation, e.g. the execution of an oxidation step, the deposition of a layer, the application of a photomask, exposure, developing, etching, and/or ion implantation.
By way of example, to fabricate what are known as bipolar complementary metal oxide semiconductor circuits (BICMOS circuits), it is necessary to protect semiconductor structures in one base technology, while the semiconductor structures of the other base technology are being produced. Under certain circumstances, a repeated change between the regions that are in each case to be protected and the regions which are in each case to be processed may be necessary.
It is accordingly an object of the invention to provide a method for fabricating a semiconductor structure using a protective layer, and a semiconductor structure which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which provides an improved method for fabrication of the semiconductor structure using a protective layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fabrication method. The method includes providing a given structure being either a substrate or a semiconductor structure; applying a protective layer to the given structure; carrying out at least one intermediate process and the intermediate process leading to a formation of cracks in the protective layer resulting in an impairment of a protective action of the protective layer; applying a repair layer to the protective layer; and removing the repair layer before an application of further layers and/or before carrying out subsequent process steps.
The invention is based on the consideration that the known protective layers in principle ensure sufficient protection to, for example, avoid damage to the semiconductor structure that is to be protected. However, the protective layer is usually applied to a topological surface, i.e. to an uneven surface, for example with height differences of over 100 nm. While the intermediate processes are being carried out, the chemical and mechanical properties of the protective layer are influenced.
By way of example, heat causes a shrinkage of the protective layer, so that tensile forces are formed in the lateral direction. The tensile forces lead to the formation of shear forces and possibly also of notch effects in particular at the locations at which height differences occur in the protective layer. On account of the lateral forces, cracks and gaps may form in the protective layer.
By way of example, on a wafer with a diameter of, for example, 6xe2x80x3 or 12xe2x80x3 (inches) only three or four such cracks are formed. The cracks are highly disruptive in particular because they reduce the effectiveness of the protective layer, so that components that are unable to function are fabricated or long-term reliability problems arise. Deposits that penetrate into the crack often only diffuse very slowly to the semiconductor structure, where they lead to delayed failures that only occur in the end product. The crack is, for example, only a few nanometers wide and only a few nanometers deep. The length of the crack is, for example, a few nanometers, a few tens of nanometers or a few hundred nanometers.
Moreover, the invention is based on the consideration that only with great difficulty is it possible to reduce the number of cracks, since measures aimed at avoiding the formation of cracks are highly complex.
Therefore, in the method according to the invention, a protective layer that is of a thickness at which cracks occur during the intermediate processes is applied. In other words, the formation of cracks is accepted in the method according to the invention. However, in the method according to the invention, a repair layer is applied to the protective layer whose protective action has been impaired, and the repair layer increases the protective effect of the protective layer again in such a way that the repaired protective layer once again effectively protects the semiconductor structure which is to be protected during subsequent processes.
In the method according to the invention, the repair layer is removed again, for example, by a fixed-time etch or an end-point etch, before further layers are applied and/or before the subsequent processes are carried out, in order for the substrate to be processed further. However, residues of the repair layer remain in the cracks in the protective layer and also, in the case of relatively deep cracks, in the semiconductor structure which is to be protected or in a substrate which is to be protected. The residues in fact increase the protective effect of the protective layer. Moreover, it is only possible for residues of the repair layer to remain in the cracks in the protective layer if the protective layer has not been removed during removal of the repair layer.
Therefore, the result of the use of the method according to the invention is that even with a simple procedure, i.e. when a relatively thin protective layer is applied and, for example, when a protective layer which contains only one material is applied, a high protective action and therefore a high yield are nevertheless achieved after the repair. In particular, the particularly critical delayed failures are avoided. The application of a further layer with a complete protective action is dispensed with.
In a refinement of the method according to the invention, the thickness of the repair layer is significantly less than the thickness of the protective layer. By way of example, the thickness of the repair layer is less than approximately one-third of the thickness of the protective layer, less than approximately one-tenth of the thickness of the protective layer or less than approximately one-hundredth of the thickness of the protective layer. A repair layer for repairing a 100 nm (nanometer) thick protective layer has a thickness of, for example, 20 nm.
In one configuration, the thickness of the repair layer is half the maximum crack width, so that even the crack that has the greatest crack width is just still reliably filled. When all the cracks have been filled, the deposition of the repair layer is ended without the thickness of the repair layer being increased still further. The application of such a thin repair layer can be carried out quickly and easily. Moreover, a thin repair layer can be removed more easily than a thicker repair layer.
In a further refinement of the method according to the invention, the repair layer contains the same material as the material of the protective layer. The result of this is that the protective layer and the deposits of the repair layer that are present in the cracks have uniform etching properties. In this way, it is possible to effectively prevent the cracks from being etched open during subsequent etching operations. In one configuration, the repair layer is applied using the same process as the protective layer. This measure too leads, for example, to uniform etching properties of protective layer and repair layer. By way of example, the protective layer used may be TEOS (tetraethyl orthosilicate) which, by way of example, has been compacted or consolidated. In this case, TEOS is also used to repair the protective layer and is then likewise compacted.
In an alternative refinement, the repair layer is formed of a different material than the material of the protective layer. Degrees of freedom resulting from this measure make it possible to use layers of materials that also serve to fabricate structures in the unprotected regions of the substrate as the repair layer. In one configuration, the repair layer is applied using a different process than the protective layer, in which case the repair layer may optionally be formed of the same or a different material than the protective layer. This measure too brings degrees of freedom that make it possible for processes for applying or removing the repair layer subsequently also to be used to fabricate the structures in the regions that are not protected by the protective layer.
In a further refinement of the method according to the invention, the protective layer protects a field-effect structure, i.e. a structure whose operation is based on the field effect, e.g. a MOS (metal oxide silicon) structure. The structure includes, inter alia, a gate electrode, a gate insulation layer, a spacer and, if appropriate, also a lightly doped drain (LDD) connection region for source and drain regions which are themselves relatively highly doped, i.e. what are known as highly doped drain (HDD) regions. The method according to the invention is particularly suitable for protecting the entire gate, in particular for protecting the gate insulation layer. The gate insulation layer is only a few nanometers thick, and consequently it is highly sensitive, in particular including with regard to the diffusion of impurities into it, which impurities could reach the vicinity of the gate insulation layer through the unrepaired cracks.
In a further refinement of the method according to the invention, the further semiconductor structure is a bipolar transistor. In particular, for the fabrication of BICMOS structures, protective layers that offer effective protection are required. The use of the protective layers results in degrees of freedom in the process management which make it possible, by way of example, first to fabricate the buried layers for the collector connections of the bipolar transistor, then to completely or at least partially fabricate the MOS structures, and to fabricate the bipolar transistor further only after a protective layer has been applied to the MOS structure.
In a further refinement of the method according to the invention, conformal deposition is used to deposit the repair layer, the process is able to completely fill gaps or cracks, i.e. without leaving any cavities. The conformal deposition is achieved with a reaction-limited deposition process, i.e. a deposition process in which, unlike with a diffusion-limited deposition process, the deposition rate is limited by the reaction conditions, in particular by the temperature, but not by a deficit of particles that are to be deposited. A completely filled crack increases the protective effect of the protective layer considerably more successfully than a crack which is only partially filled and still includes a cavity, i.e. what is known as a void.
In another refinement of the method according to the invention, the protective layer is a silicon dioxide layer or a TEOS layer. Silicon dioxide or TEOS is like-wise used as material for the repair layer. The materials are easy to apply and remove again with the aid of etching processes.
In a further refinement, the semiconductor structure that is to be protected includes spacer structures for the implantation. The spacer structures are fabricated, for example, from silicon dioxide or from TEOS. Alternatively, however, it is also possible for nitride to be used as material for the spacer structures.
In a further refinement of the method according to the invention, the protective layer is removed again after the protective layer has been repaired and the subsequent processes have been carried out. The result of this measure is that the semiconductor structure that is to be protected can be processed further at a relatively late point in the overall process, for example, by forming a silicide layer. To remove the protective layer, it may also be necessary to remove the repair layer or a layer that has been applied to the repair layer. Cracks that penetrate through the protective layer remain closed even after the protective layer has been removed, for example, on account of an isotropic etching process being used to remove the protective layer.
In another refinement to the method according to the invention, the intermediate process is carried out at a temperature between approximately 700xc2x0 C. to approximately 900xc2x0 C. These temperatures often lead to particularly strong shrinkage of the protective layer as a result of thermal compression, for example, in the case of TEOS protective layers. Therefore, there is a particularly high risk of cracks being formed.
In a refinement, at least one subsequent process is such that without application of a repair layer the semiconductor structure that is to be protected by the protective layer would be damaged when subsequent processes are carried out, on account of the reduced protective effect, and/or their function, in particular their long-term function, would be impaired. In other words, the subsequent process extends all the way to the protective layer. On the other hand, the repair layer does not have to provide a sufficient protective effect with respect to the subsequent process on its own, and can therefore be very thin compared to the protective layer. A sufficient protective action results from the interaction of protective layer and repair layer or of the protective layer and residues of the repair layer in the cracks. By way of example, without a repair, it would be impossible to use an isotropic etching process to remove the protective layer, since the etching takes place significantly more quickly in the region of the unrepaired cracks. Also, a structure lying below the protective layer could no longer reasonably be used as an etching stop layer in a selective etching process for removing the unrepaired protective layer, since the etchant bypasses the etching stop layer through the cracks. A further example of a subsequent process which extends as far as the protective layer is a furnace process in which, although contamination and/or impurities penetrate through the thin repair layer, they are stopped by the protective layer or by the material of the repair layer which is present in the cracks.
Moreover, the invention relates to a semiconductor structure that includes a crack that has been repaired. In refinements, the semiconductor structure has been fabricated using the method according to the invention or a refinement of the method according to the invention. Accordingly, the technical effects that have been described above also apply to the semiconductor structure and its refinements.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a semiconductor structure. The semiconductor structure contains a substrate, at least one component structure applied to the substrate and has at least one crack formed therein, and a repair layer having a repair material filling the crack. The repair layer is removed again before further layers are applied and/or before subsequent processes are carried out.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a semiconductor structure using a protective layer, and a semiconductor structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.